1. Field of the Invention
The present invention relates to a display apparatus, and more particularly to a backlight unit in a liquid crystal display.
2. Discussion of the Related Art
A liquid crystal display (hereinafter abbreviated LCD) includes a panel part, a driver, and a backlight part, thereby displaying a video by changing molecular arrangement of liquid crystals in the respective pixels by applying electric power to the electrodes installed at both upper and lower glass plates between which a liquid crystal is injected.
Unlike a cathode ray tube (CRT), a plasma display panel (PDP), and a FED, a liquid crystal display cannot be used without an external light because it is non-luminescent. For the purpose of overcoming this weak point and working in the dark, a backlight that applies light uniformly to an information display face of the LCD is used.
Backlights are mainly divided into three categories: (1) a vertical type, in which a lamp irradiates light to a front face from a rear of the liquid crystal display; (2) a side type, in which a lamp is placed at a lateral side of a light-guiding plate through which light is applied to a front face of the LCD; and (3) a wedge type, which is included in the side type, in which a light-guiding plate is slanted.
Reference will now be made to the components and operational principles of the backlight, an example of which is illustrated in the accompanying drawing.
FIG. 1 shows a cross-sectional view of a general side type backlight unit.
Referring to FIG. 1, light emitted from a lamp 11 enters a light-guiding plate 12. In this case, most of the incident light is transferred to the end of the light-guiding plate 12 by total reflection. However, some of the light entering under a specific incident angle, e.g., a direction vertical to a light-guiding plate plane, is not totally reflected, but transmitted through a reflection plate 13 or propagated to a diffusion plate 14.
The reflection plate 13 reflects the light that passes through a rear of the light-guiding plate 12 to the light-guiding plate 12. The light having passed through the light-guiding plate 12 is dispersed, thereby requiring a diffusion plate 14 to provide a uniform brightness of the dispersed light. The brightness of the light having passed through the diffusion plate 14 is then reduced abruptly. In order to increase the reduced brightness, the light is converged by a horizontal/vertical prism sheet 15 so as to propagate into an LCD panel.
A lamp is one of the most important parts of the above-structured backlight unit. There are a number of different lamp types, including EL (electroluminescent) lamps, LEDs (light emitting diodes), CCFL (cold cathode fluorescence lamp) and the like.
Lately, LEDs have drawn attention for use as a backlight of an LCD. An LED operated by the voltage of DC 5V has a life span longer than that of EL or CCFL. In addition, such LED needs no extra inverter. However, an LED requires a current control circuit to protect the LED. Also, an LED placed below an LCD panel generally emits yellow-green color, but can be enabled to apply various colors.
Reference will now be made to the structure of the backlight in detail according to a related art, an example of which is illustrated in the accompanying drawing.
FIG. 2 shows a layout of a lamp in a vertical type backlight unit according to a related art.
Referring to FIG. 2, LED lamps representing R, G, and B (red, green, and blue) colors, respectively, are arranged in order.
A lamp of a vertical type backlight unit applicable to a field sequential LCD having said arrangement is turned on/off by the following sequence.
The lamp is driven by the sequence: 1st line R→2nd line R→3rd line R→1st line G→2nd line G→3rd line G→1st line B→2nd line B→3rd line B.
Unfortunately, LCD according to the related art has the problems as follows.
First, as various LEDs, each of which represents only one color are used in combination, each interval between the lamps having the same color is so far that it is impossible to avoid leaving a long distance between a backlight and a diffusion plate to maintain uniform brightness. Therefore, the backlight unit tends to be thicker.
Second, several diffusion plates are required for reducing the interval between the backlight and diffusion plate. Therefore, brightness of the backlight is reduced by using too many diffusion plates.
Third, circuitry of a printed circuit board (PCB) substrate to which the lamps are fixed becomes complicated.